1. Technical Field
The present invention relates to a driving method of a liquid crystal device.
2. Related Art
During the driving of the liquid crystal device, an alternating current (AC) driving in which the polarity of the voltage applied to the liquid crystal element is repeatedly reversed is generally used to suppress degradation of the liquid crystal device caused by the remaining direct current (DC) components. As a method of the AC driving, JP-A-2002-196358 discloses a driving method in which an output range of the signal line driving circuit for writing an electric potential to pixel electrodes of the liquid crystal elements is reduced by supplying signals to signal lines and the voltage applied to the liquid crystal element is set to a desired voltage by altering the electric potential written to the pixel electrode using a capacitive coupling to suppress power consumption. Hereinafter, this driving method is called “capacitive line driving.” In this capacitive line driving disclosed in JP-A-2002-196358, the polarity of the voltage applied to the liquid crystal element is reversed whenever the electric potential is written to the pixel electrode.
In addition, as a method of driving a liquid crystal device, a subfield driving is known in the art, in which a single frame is divided into a plurality of subfields, and multiple gradations are displayed on the liquid crystal element by applying either one of two voltages (absolute values) to the liquid crystal element in each subfield. JP-A-2003-114661 discloses, as an example of the subfield driving, a driving method in which the lengths of a plurality of subfields included in a single frame are different in order to reduce the number of subfields. Hereinafter, this driving method is called a “weighted subfield driving.”
Herein, to obtain the advantages of both the capacitive line driving disclosed in JP-A-2002-196358 and the weighted subfield driving, it is assumed that both driving methods are combined. In this case, if the number of subfields per single frame is even, a problem occurs. This problem will be described with reference to FIG. 28. In this description, the liquid crystal device obtained by combining both driving methods is called a “liquid crystal device in the related art.”
Referring to FIG. 28, voltage patterns applied to a normally black type liquid crystal element are illustrated across the (k)th frame and the (k+1)th frame in the case where, in the liquid crystal device in the related art, the number of subfields per single frame is set to 4, and a total of 16 gradations, from the 0th gradation to the 15th gradation, are displayed. For expressions corresponding to each gradation, there are shown comparative expressions between a time integral (absolute value) of positive voltages and a time integral value (absolute value) of negative voltages applied to the corresponding liquid crystal element 40 in the case where the gradations are displayed across the (k)th frame and the (k+1)th frame.
According to the capacitive line driving disclosed in JP-A-2002-196358, polarity of the voltage applied to the liquid crystal element is reversed whenever an electric potential is written to the pixel electrode. Meanwhile, according to the subfield driving, the writing of electric potentials to the pixel electrode is performed for each subfield. Therefore, in the liquid crystal device of the related art, as shown in FIG. 28, the polarity of the voltage applied to the liquid crystal element is reversed for each subfield. In other words, the subfield for which the polarity of the voltage applied to the liquid crystal element is positive (+) and the subfield for which the polarity of the voltage applied to the liquid crystal element is negative (−) are alternately arranged.
Therefore, the polarity relating to the subfield within the (k)th frame and the polarity relating to the subfield within the (k+1)th frame having the same length as that of the corresponding subfield are opposite to each other when the number of subfields per single frame is odd. Otherwise, if the number is even, the polarity is not opposite. For example, when focusing on the 7th gradation, the applied voltage relating to the subfield SF1 is set to +5 V even in both the (k)th and (k+1)th frames. Therefore, when the number of subfields per single frame is even, the comparative expressions become inequality expressions except for the 0th gradation, as shown in FIG. 28.
This means that the DC components remain except for the 0th gradation. The remaining DC components accelerate the degradation of the liquid crystal element.